1. Field of the Invention
The present invention relates to a light guide plate of a surface light source used in a liquid crystal display (LCD), and particularly to a surface light source having a light guide plate with prisms configured to provide uniform illumination.
2. Prior Art
In general, LCDs have two main advantages in comparison with cathode ray tubes (CRTs): LCDs are thin, and have low power consumption. It has been said that LCDs might one day completely replace CRT display devices, and LCDs have aroused great interest in many industries in recent times. In general, an LCD needs a surface light source to provide even light for a clear display.
A surface light source comprises a light source and a light guide plate. The light source may be a linear light source, or one or more point light sources. The light guide plate has an end face through which light is introduced, and two opposite major faces one of which functions as an emission face. The performance of the surface light source greatly depends on the characteristics of the light guide plate employed therein.
A light guide plate functions to change a direction of propagation of light beams emitted from the light source and introduced into the light guide plate, from a direction roughly parallel to the emission face of the light guide plate to a direction perpendicular to the emission face. That is, the light guide plate effectively changes the linear or point light source(s) into a surface light source, for evenly illuminating a whole display screen of the LCD.
FIG. 4 shows a conventional surface light source 1, which comprises a reflector 12, a light guide plate 13, a diffuser 14, a brightness enhancing film (BEF) 15, and two point light sources 11. The reflector 12, the light guide plate 13, the diffuser 14 and the BEF 15 are stamped together from bottom to top in that order. The point light sources 11 are disposed adjacent an incident surface 131 of the light guide plate 13. The BEF 15 has a plurality of parallel prisms 151 evenly disposed on a surface (not labeled) thereof. In use, light beams from the point light sources 11 enter the light guide plate 13 through the incident surface 131, and then are introduced into the BEF 15 in a direction perpendicular thereto. The BEF 15 converges the light beams in a predetermined direction to evenly illuminate a whole display screen of an associated LCD.
FIG. 5 shows an essential optical intensity distribution of the light beams in the light guide plate 13. FIG. 6 shows an essential optical intensity distribution either one of the point light sources 11. Each point light source 11 emits light beams over a limited predetermined range of angles, and the light beams enter the light guide plate 13 with an uneven distribution. As a result, three dark areas 261, 262, 263 are created in the light guide plate 13. The luminance of the dark areas 261, 262, 263 is less than that of a remaining main area of the light guide plate 13. The surface light source 1 does not attain uniformity of light beams exiting therefrom.
In addition, the surface light source 1 comprises the plural above-described elements, including the diffuser 14 and the BEF 15, for enhancing brightness. This adds to the size and the cost of the surface light source 1.
A new surface light source and a light guide plate for the surface light source which overcome the above-mentioned disadvantages are desired.